Hey all, Ive been trying to figure out why my c30-74 with 75mm pulley is not producing as much boost as expected (compared to similar setups)

I got the compressor second-hand from a kraftwerks kit, it had around ~20k km with no track use. Using the 85mm pulley the whole time.

I have an EUDM NBFL with a decently healthy fully stock motor (~155psi compression on all 4, very little blowby and oil usage (cant measure any on dipstick after 2h of track time), about 100ml use per 1000km for city driving)
Setup:
Racing Beat header
Racing beat resonated race midpipe
Custom 2.5" ID straight through muffler
Large 700x230s63mm Intercooler
2.5" coldside piping
2" to 1.75" hotside piping
Bosch/kayser 25mm recirc valve (the common VW/Audi valve)
ID1050X Injectors
Rev MS3Basic


I have soft rev limiter (progressive spark retard and progressive spark cut) starting at 7100, with hard cut at 7200

With the 85mm pulley I was usually achieving about 0.52bar/7-8 psi at 7100rpm (in very cold days, 0.62bar @7200), with this 75mm pulley I am hitting about 0.74bar / 10-11psi) @ 7100rpm

The first thing to be checked was belt slip, I installed a Gates RPM belt which has much better traction and is much stiffer than normal belts, with this belt I dont see much dusting (just a slight mist after a track even).
I also have a very VERY stiff custom bracket which eliminates tension loss due to flexing compared to the default craptastic aluminium and thin kraftwerks bracket.
I also seem to have a good wrap around the compressor pulley and the auto-tensioner is very far from hitting the stops:


The next step was to check for leaks. I hooked up some PVC caps to the compressor inlet and to the hose before the throtle body to isolate the pre-TB piping. I used a high volume bycicle pump to inflate it.
Found some minor leaks that could be easily heard, simply tightening the hose clamps a bit more with a box wrench instead of just using a screwdriver solved all the leaks.
I pressure tested this whole system easily up to 2 bar / 30 psi, with no pressure loss after 30ss. (before I was unable to manually pump above 0.4bar without serious effort)

This had no effect on peak boost, still reaching 0.74bar / 10.7 psi at 7100 rpm

The next step was to pressure test the system including the intake manifold (squaretop)
I found a leak in the EGR valve body (I had the hole where the pipe connects already plugged, no leaks there), so I made a plate to delete the valve

I also noticed the PCV was leaking pretty badly, from completely unpressurized system, one pump of the pump would immediately start leaking a good amount of air through the valve. I dont have any healthy valves, so I just capped the manifold hole securely and left the pcv unhooked.

I then re-pressurized the system to a bit over 1 bar / 15psi and was unable to hear any leaks anywhere (in a quiet underground garage). It took about 20s to drop the pressure to 1-2psi, obviously due to cylinder/valve leakdown. It did not take much effort to manually pump all this to 1 bar, so I believe there are no further leaks.
The recirc valve was hooked up to the bottom side (pressure directly on the diaphragm and the pressure reference line was also hooked up to the manifold, it did not leak)

Went for a ride, seeing basically same pressure of 0.74 bar at 7100 rpm in a second gear pull, in 3rd I was able to peak at 0.77 bar / 11 psi

Ambient temperatures roughly 19-20C for the past 2-3 days during all tests, but even testing at 18C and 23C during the day barely makes a difference (at best 0.01 bar difference)

To be triple sure I had no obvious belt slip or weird boost leaks, I used MLV's custom field feature to plot boost in relation to RPM, the function is:
(([Field.MAP]-[Barometer])/100) / ([Field.RPM] / 1000)

This will roughly show how many bar of boost I am producing per 1000 rpm

The graph is pretty consistent upward slope with no obvious plateaus or drops (field is `boost x rp,`):


(2nd and 3rd gear pulls, the test road has a few bumps that make RPM vary a bit, but the trend is obvious)
(In 3rd I stayed on the limiter a bit so I managed to peak at 180kpa, but thats not realistic, place to measure is just before the first timing reduction at 7100rpm)

Miscelaneous other things I have tested:
Running with no recirc valve (to ensure valve is not leaking, theory being if the hotside was restrictive the higher pressure would push the valve open as the spring and the manifold pressure would be unable to keep it shut) - No differece, still 0.74 bar at 7100rpm
Running with recirc valve inverted (pressure hitting side of diaphragm instead of bottom) - No difference
Setting VVT advance to 0* at redline (at redline im at about 3-4* of advance, consistently dyno tested to produce a bit more power NA and with the Rotrex) - No difference, EGO started reducing fueling consistently by 1%
Straight Silicone hose to air filter to rule out kraftwerks multi-hole inlet piece (air filter, compressor inlet, recirc valve, breather port), a simple 2" silicone pipe (assumption was that it was acting as a restrictor) - No difference, the actual inside diameter of the silicone elbow is the same as the ID of the rotrex.

Why do I think I should be making more boost?

There are a few cars with almost identical setups, making about 0.9 bar / 14 psi at redline.

Locally, there is another NBFL using the same bracket and pulley, same original kraftwerks 1.75" hotside piping, 2.5" coldside, but with smaller intercooler, RB exhaust except usinng OEM NBFL tubular manifold.
In this forum at least a few people from previous searches seem to be hitting 13-14 psi with no issues:
- Creampuff hitting 14psi with 75mm pulley
- Mobius had a CNC head and also a case of "missing boost" - until the accident and then he got an EFR
- A few others, including people making ~10psi with 80mm pulley ( @Lincoln Logs , @tomiboy , @dcamp2 et al), I believe most of them with 1.75" hotside

I dynoed with 85mm pulley - 230hp @7200rpm, 230NM @ 6000RPM - around 17C ambient temperature (obviously values are corrected for temperature and pressure, and for the dyno I use, drivetrain losses)
With 75mm pulley and 23C ambient, I got 250hp @ 7200RPM and 250NM @ 6200RPM - running about 21* of timing at redline on 98RON for both pulleys, 11.5 AFR

Aforementioned local NBFL on the same dyno ( ~15C ambient temp) got 265hp @ 7000RPM (limiter set there) and 290NM @ 6000RPM.

(for some context/referene I got about 158hp N/A with same exhaust, stock injectors, 31* timing at redline and the typical 21" freeflowing "supermiata" intake. With ITBs and everything else the same I got 173hp)

I dont fully expect to reach 0.9 bar / 14 psi in normal day temperatures (~23-24C) but would expect easy 0.8-0.84bar / 12-13 psi.

 

85/75 = ~1.13

(7+14.7) x 1.13 = 24.5
24.5 - 14.7 = 9.82

 

Yes ofc the change from 85mm to 75mm has a roughly predictable increase, but even when I was using the 85mm pulley boost was inferior to what others reported

I that case ((9.7+14.7) * 1.13) - 14.7 = 12.8psi

 

I'm at VIR right now. It's hotter than my *****. Barometric pressure is 29.49. With the 80mm pulley, I'm getting 8.7 or so at around 7000. I did a lot of short shifting due to the heat yesterday, but one of my logs shows 8.4 psi at 6800. Ambient temp was around 96.

 

Thanks for the info. Those seem like numbers id make when its super hot out (>30C)

I forgot to mention but I'm roughly at sea level, around 101kpa where I test the car

 

Wow, first off. Great job on posting logs and providing legitimate data to work with. Have you check the gaskets on the intake manifold? What about the fuel injector o-rings? When I was tracking down the last couple psi of boost I found both of those as culprits. The intake manifold in particular uses a metal gasket that can get tweaked if you are not careful. I am running a 2" hot side, but that really isn't much different than the 1.75"



I make about 9.5-10psi peak with the 85mm pulley @ 7100 RPM, I never switched to the 80mm, I was able to hit about 260whp with E85. Plenty for a stock motor. I do see some fluctuation in peak boost depending on weather conditions and location, maybe .5psi or so. Here is a snapshot of a log from my car doing a pull. This one made 166kpa, ~9.5psi. I have tuned another local car running a C30-74 on a 1.6 Miata with a 75mm pulley, he made 199 KPA. Boost wise you are down, but I think the numbers you made on the dyno on the ball park. If the car is running well, and you are certain there are no leaks it may just be a slight variance in the blowers. I'd run it and enjoy.

 

What elevation are you at?

 

I think Sean is on the money here and you are getting to hung up on boost numbers.

Sc setups will have some boost variances with different intake/exhaust setups.

Sounds like the power is there, and you did a great job diagnosing.

 

Keep in mind that ~boost~ in the supercharger world is a pretty meaningless number. SCs work on flow ratios. SC inlet flow divided by engine inlet flow equals pressure ratio. An engine that flows more air, due say to a superior volumetric efficiency will have a lower PR (or boost). If the dyno numbers are similar between setups, chances are the SC inlet flow is the same. It works the other way too. A SC that has lower inlet flow due to lower SC VE will have a lower PR.

Certainty investigate for leaks as mentioned above, but if you have a head that is flowing better or a better exhaust, boost will go down.

Edit: minor correction for accuracy and grammar

 

Thanks Ted!
I didn't want to create a thread until I had tested everything I feasibly could, to avoid multi-day back and forth of frustration for everyone.
Yeah the numbers are on the ballpark so in that regard Im good, its nearing the limits of the 5 speed (that I want to retain as long as possible for that slick feel) and the stock internals, so I have good safety margin to abuse it on track.
I reconfirmed with the local rotrex and he was making 0.9 bar at 7200-7300, but he had a cat and less optimal IC than now, AFAIK he is now making less boost but I have not been able to get a log to confirm

As for leaks, I validated that none of the injectors are leaky or the gasket (nothing that matters), I actually thought it was the number 4 injector at first, but then confirmed it was the EGR with soapy water.
I also doused the intake gasket with soapy water and no visible bubbling was seen (or hear with my ear against the top of the manifold)
There are some miniscule leaks in some places where the halves of the squaretop meet, but they are almost invisible even with soapy water at 20psi and completely inaudible even with my ear right next to them. I also had a tiny leak in the tube that connects the two halves near the TB and fixed it with a ziptie.

As for being hung up on boost, it was mostly a fear of something being wrong, if I ever considered swapping to a C30-94 or a turbo setup I was afraid something more fundamental was wrong which could cause issues

I did have a look at the 30-74 compressor map but, ignoring the fact I dont know how much pressure I'm making at the outlet, if Im really making sub 0.8 bar that would mean I'm way outside the map in compressor choke area (although in a rotrex there is no danger of overspinning ), so Im likely outflowing it? I dont have a notion of how much .30kg/s of flow is, I'd have to compare to some turbo maps from acquaintances with the same engine.
This area? (green line extrapolates the 120k rpm line):



I was also surprised I could manually pressurize the whole system and not leak from valves and pistons that much!

I might swap to 2" hotside just for consistency and more efficiency maybe, and also hardware availability (not to mention that without T-clamps my metal clamps are almost stripping to get them tight enough to not leak, or have to be placed OVER the bead and not after it to spread the load over a smaller surface area)

I will have to dyno the car again after fixing all the leaks and maybe swap to 2" before then.

I think if we had E85 here I would not run this pulley, I would definitely be at an unsafe level for stock motor and 5speed (although...I do have a flex fuel sensor working right now :P)

 

I know it sounds like a nit pick, but a little leak here and a little leak there does add up. I like using the old school approach tracking down the final leaks spraying carb cleaner on suspect areas. If it's enough of a leak to suck up some carb cleaner and make the idle droop, it's enough to lose boost.

Assuming it's not sucking up and carb cleaner, then it's just how your set up is. Intercooler pressure drop could have something to do with it too. Like I said before though, you are in the ball park I wouldn't fret over it.

 

Boost is a measure of backpressure through the engine so less of it at the same flow rate actually shows greater efficiency, unless that flow is escaping somehow. Different configurations or even different engines in the same family can have some variation. Cam timing, valve lash, head design, and exhaust system inequalities can keep yours from being exactly like your neighbor's.

Even different air filter arrangments can vary the restriction and skew the results between motors.

 

Yep, at this point I think Im just flowing a bit better. Also the other car was tested at very cold temperatures (maybe 12-13C) which just by itself almost makes up the whole difference.

As for my question about the compressor map, am I reading it right, being in the choke area?

Ill test with carb cleaner soon around the manifold.

 

Checked Idle with Carb cleaner, no change dousing IM half-joints, fange, IAC, etc.. Seems no more real leaks.
Tested with 18C ambient temp, reaching 0.77bar / 11.1psi @ 7100rpm.

Seems its good, will have to dyno at some point

 

Same problem here. I only get 7,5 psi at 7000 RPM with a 80 mm pulley, but I'm just at the beginning of diagnosis. Boost leak check is planned for the weekend.

Where did you pick up the Map-pressure? Can there be a difference between various positions on the intake manifold?

 

Going to need way more info on the setup.

Honestly, I would just get the 75mm pulley either way, on 93 (98RON) its easily within safe 5 speed and stock motor torque levels.

Report back when you have leak tested the whole thing as well.

 

Issue fixed. I found four boost leaks (oblique clamp on a boost pipe, EGR connectors and charcoal canister solenoid)

Now I get 9 psi, which should be ok for my elevation (1900 ft).

For now I stay with the 80 mm pulley, because I'm still using the OEM header and cat.

 

Replaced all hotside tubing with 2". a lot less curves too, overall just cleaner and better diverter location.


90 degree silicone, 45 degree silicone, 90 degree aluminium pipe into IC.

Unfortunately its been a little colder at night recently so direct comparison is hard (about 16-17C ambient vs 20-21 for previous tests)
I was hitting 0.82 bar at 7100rpm, peak of 0.85bar.
Seems good, not really much left to do, and at a good power level. Plainly for optimization reasons would love to have the intake receive ambient air from somewhere, but dont really know a good way to get that. I have the cappucino water bottle in place so id have to rig it further inboard to get air from the cowl area...but then id have to build a sealed air box so as to not increase under hood pressure.
I think ill start another thread for that purpose.

 

Highly doubt “cold air” is gonna net you more power. That’s the intercooler job if it’s sized accordingly. If I was you, I’d change out the sc inlet for 3 inch if it already isn’t.

 

Its mostly a small optimization, not going to net me gobs of power. Ive measured average 10C above ambient on track (23C ambient, 33C for most of the lap, 40C at the end of a main straight). I see about 2-3C above ambient when cruising (I dont get any real ambient air into the bay, since I have good ducting). Lower temps are better in general and if the optimization doesnt take major effort and funds, its worth doing IMO.

The c30-74 inlet is 2"/51mm ID, the 90 degree elbow ID matches it perfectly and the filter and pipe are at least 2.5" (don't recall). Dont know if the tubing is a real restriction

After searching for a while on this site, It seems the actual filter might be a bigger restriction that the short *** pipe.
Based on @patsmx5 thread , even If Im obviously pushing way less air it might be advantageous to get:
a) a bigger filter
b) a type of bellmouth/velocity stack to adapt to the filter

a is easy enough to fix, and I do have decent room to get a longer filter and hide it near all the way up to the clutch master
b, after some search, doesn't bring out many results, (especially with easy availability shipped to europe at decent prices)

Even in Pats' thread one thing he also mentions, following my reasoning too, is that with a turbo even if your pre-compressor setup is not ideal, you will never notice due to the wastegate boost control, with a SC the rpm is set so any gains and losses should have a much greater effect.

I would actually find some fun in measuring the restriction, but I dont have a spare MAP sensor. I do have a china differential pressure gauge I can use, maybe that would work? (Doesnt do datalogging but I could just film it or just check max numbers)
From some quick reasoning, id have one sensor inside the tubing pre-compressor and the other to just outside the filter, does that sound right?

Should I start another thread for filters? All the older threads I can find for this site have pretty sparse info besides Pats' thread and sixshooter basically recommending the RU-5111 in every thread